: If the radiation is targeted to specific sites, specific irradiation damage can more efficiently kill targeted tissues or cells. Specific irradiation is achieved using 125I decay to locally irradiate nanometer volumes in cells by the emission of many low energy Auger electrons. Estrogen receptor expressing cells and cancers can be targeted with an estrogen ligand having a radionuclide (125I or 123I) covalently bound to the estrogen. Preliminary studies indicate 125I-estrogen decay induces 10 times more DNA damage than when 125I is randomly incorporated into a central position between complementary strands of DNA using the thymidine analog, 125IUdR. We propose that the increased DNA damage induced by 125I-estrogen is explained by the non-random placement of the 125I-estrogen at the nuclear matrix associated with the bases of several looped domains of DNA. DNA damage at the base of the looped DNA domains should produce clustered DNA damage in the size class range of looped domains of DNA (10-200 kb DNA) and smaller. The specific aims of this project were designed to confirm and explain these results. We propose use of conventional agarose gel electrophoresis to detect the presence of short fragments of DNA produced by the decay of 125I-estrogen. Clustered DNA damage induction results will be confirmed in another cell line, human breast cancer cells, MCF-7 and will be confirmed using another Auger emitting radionuclide, 123I-estrogen, to damage DNA and cause cell killing. Finally, the estrogen itself can not be acting as a radiosensitizer. The broad, long-term objectives are to determine the role of chromatin organization in radiation induced cell death.